Marshal.dump corrupts Hash data when serializing

=begin
Marshal.dump seems to corrupt a Hash object during serialization.
The problem is that this does not happen when the affected Hash is serialized by itself but only (as far as I could see) when it's part of a larger object.

Worked on: ruby 1.9.3p374 (2013-01-15 revision 38858) [x86_64-linux]

Unfortunately, it can't be reproduced using the attached serialized objects, it only happens when I run one of my system's tests.

I thought that I might be doing something wrong but it works fine on 1.9.3 and also works on 2.0 when I change the serializer to YAML.